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Hydrodynamic and geomorphological controls on suspended sediment transport in mangrove creek systems, a case study: Cocoa Creek, Townsville, Australia
Estuarine, Coastal and Shelf Science 56 (2003) 415–431
Hydrodynamic and geomorphological controls on suspended sediment transport in mangrove creek systems, a case study: Cocoa Creek, Townsville, Australia S. Brycea,*, P. Larcombea, P.V. Riddb a Marine Geophysical Laboratory, School of Earth Sciences, James Cook University, Townsville 4811, Australia Marine Geophysical Laboratory, School of Computer Science, Mathematics and Physics, James Cook University, Townsville 4811, Australia
b
Received 15 February 1999; received in revised form 8 July 1999; accepted 25 February 2002
Abstract In tide-dominated sedimentary systems, close relationships exist between tidal hydrodynamics, sediment transport and geomorphology. Tropical coastlines contain many tide-dominated mangrove creeks, yet few studies to date have examined the detail of such relationships for these environments. Time-series observations of tidal height, currents and suspended sediment concentrations were taken between 1992 and 1996 in Cocoa Creek, a mangrove creek system near Townsville, NE Australia. The creek and surrounding mangrove swamps and salt flats were surveyed with an echo-sounder and total survey station, respectively. For Ôwithin-channelÕ tides, the flood tide is always the fastest, at up to 0.5 m s1. In contrast, for overbank tides (i.e. tidal height > þ1.5 m Australian Height Datum, AHD) ebb currents are fastest in July, December and January, but flood currents are fastest in August and September, at up to 1 m s1 in both cases. The tidal asymmetry of overbank tides in Cocoa Creek is controlled by the interaction between offshore tidal forcing and the intertidal storage effect of the mangrove swamps and salt flats, with the result being that during certain periods of the year there tends to be a predominance of either faster flood or ebb velocities on overbank tides. Significant tidal suspended sediment transport in the channel is only initiated at overbank height. On overbank tides, measured net suspended sediment fluxes in the channel are mostly seaward-directed (up to 180 t per tidal cycle). However, the net flux measured over a neap–spring period may be either landwards or seawards (up to 465 and 60 t, respectively). Furthermore, on the larger overbank tides (where the maximum tidal height > þ1.85 m AHD) net sediment fluxes may be reduced because of a limited supply of available material. Thus hydrodynamic and sediment sampling durations of up to a month may not be representative of long-term trends. Given that our large dataset has not identified a clear long-term net transport direction within the creek system, we conclude tentatively that the geomorphology of Cocoa Creek may be near a long-term equilibrium. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: tidal creek; mangroves; sediment transport; Australia
1. Introduction Studies of hydrodynamics and sediment transport in mostly temperate salt marsh systems (Bayliss-Smith, Healy, Lailey, Spencer, & Stoddart, 1979; Boon, 1975; Boon & Byrne, 1981; Friedrichs & Aubrey, 1988; Pethick, 1980) have provided the initial understanding of hydrodynamic and geomorphological controls on sediment transport for tide-dominated creeks and
* Corresponding author. E-mail address: [email protected] (S. Bryce).
estuaries. Most of these studies used tidal height data to model tidal hydrodynamics, and sediment transport equations to estimate sediment fluxes and the net direction of sediment movement. Some of this work formed the basis for early attempts to understand sediment transport processes, and calculate sediment fluxes and transport rates in their tropical equivalents, mangrove creek systems (Fisher, 1994; Lessa, 1995; Lessa & Masselink, 1995; Wolanski, Jones, & Bunt, 1980; Woodroffe, 1985a). These studies employed methods such as water sampling, applying sediment transport equations and computer modelling, and they commonly assumed that sediment transport patterns
0272-7714/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0272-7714(02)00192-0
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reflected tidal asymmetry, and, further, that tidal asymmetry is similar for each spring tide period. Using these assumptions some relationships between tidal dynamics, sediment transport and geomorphology in mangrove creek systems have thus been proposed (Lessa & Masselink, 1995; Wolanski et al., 1980; Woodroffe, 1985a,b). In tidal mangrove creeks, Wolanski et al. (1980) found that on overbank tides, large water surface gradients are created due to large frictional effects caused by mangrove vegetation density, producing a shorter and faster ebb tide that helps to scour the bed of the main creek channel and thus export sediments from the creek system. Lessa and Masselink (1995) and Woodroffe (1985b) also found a similar velocity asymmetry relationship in the channel with peak flood and ebb speeds increasing as the maximum tidal elevation is increased. The magnitude of suspended sediment fluxes was found to be dependent on the maximum tidal elevation reached (Woodroffe, 1985a). However, studies which have used time-series data to actually measure sediment transport (e.g. Bryce, Larcombe, & Ridd, 1998; Furukawa, Wolanski, & Mueller, 1997; Larcombe & Ridd, 1995, 1996; Wolanski, Gibbs, Mazda, Mehta, & King, 1992a) have revealed a high degree of spatial and temporal complexity in the sediment dynamics of these tidal systems. While findings to date indicate that mangrove swamps are most likely to be places of sediment accumulation (Furukawa et al., 1997; Larcombe & Ridd, 1996), the sediment dynamics within the main channel remain unclear, and as a consequence, it is not known whether these tidal creek systems are net importers or exporters of sediment (Larcombe & Ridd, 1995, 1996; Woodroffe, 1985a). No long-term datasets of measured currents and sediment transport have been presented for mangrove creek systems. This paper details the tidal hydrodynamics, main geomorphic features and sediment transport processes of Cocoa Creek, a small mangrove creek system near Townsville, Australia. The datasets were collected over a period of 4 years and cover different temporal and spatial scales that are representative of long-term trends. Firstly, we investigate the internal and external factors responsible for the tidal asymmetry of the system, and secondly assess their effect upon suspended sediment transport as well as noting issues of local sediment availability. 1.1. Cocoa Creek Cocoa Creek is one of three tidal mangrove creek systems in the southern part of Cleveland Bay near Townsville, north Queensland (Fig. 1). The main channel of Cocoa Creek meanders for 9.5 km through a chenier plain, and extends 1.2 km seawards of the mouth across low-intertidal mudflats. The channel is fringed by a 30–150 m-wide mangrove swamp with the
largest trees (<7 m high) located at the mouth. The total catchment area is approximately 21 km2 of which 70% is made up of salt flats and mangrove swamps. There is a distinct vertical zonation of major geomorphological units in the catchment (Table 1). Annual rainfall at Townsville is approximately 1100 mm year1 of which 65% falls in the summer wet season (January–March), so that freshwater input is minimal to the creek for much of the year. The effects of freshwater runoff events during the wet season are very limited because only around 30% of the catchment sits above the supratidal zone and any significant rates of rainfall (even up to 30 mm h1) will be rapidly mixed into the strongly tidal system. During the wet season, winds are inconsistent northerlies of 5–7 kn and may only reach significantly greater speeds and variable direction when associated with the passing of tropical cyclones. The dry season (May–November) experiences consistent southeasterly winds of 10–15 kn. Tides are semi-diurnal and mesotidal with mean spring and neap tidal ranges of 2.3 and 0.4 m, respectively. Marine water initially enters the mangroves and salt flats via a network of small tributaries before overbanking of the main channel takes place at a tidal elevation of approximately þ1.5 m Australian Height Datum (AHD). The maximum tidal height reached in the Townsville area is þ2.1 m AHD. Mean sea level here is 0.1 m above AHD. At Cocoa Creek, only some spring tide periods include overbank tides, and periods of 3.5-week duration between overbank tides are common. Strong tidal currents ensure that the water column is vertically well-mixed at all times of the year. Groundwater outflow to the main channel is thought to be minimal because the surrounding salt flat stratigraphy is composed of mostly impermeable silts and clays (Ridd, Renagi, Hollins, & Brunskill, 1997).
2. Data collection and analysis Many coastal plain settings around the world, including most of northern Australia, contain extensive wetland/intertidal systems with often poorly defined catchment boundaries. Such systems may act as open drainage systems, that is water may enter and leave the system in a number of places (Fig. 1). Spatial coverage of data therefore becomes important when monitoring the movement of water and sediment within a specific creek system. 2.1. Topographic and bathymetric data Topographic surveys of the high-intertidal areas of Cocoa Creek were made to: (1) investigate relationships between the main geomorphic features and the timing of measured peaks in tidal current velocity at sites in the
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Fig. 1. Locality map and geomorphology of Cocoa Creek and the adjacent coastal plain.
Table 1 Major geomorphological features within the intertidal area of Cocoa Creek catchment and the corresponding tidal elevation range at which they occur Geomorphology Chenier/beach ridges Salt water couch meadow Salt flat Mangrove swamp Low-intertidal mudflats
Substrate lithology
Elevation range (m AHD)
Sand and gravel Clay
+3.65 to +2.00 +2.00 to +1.85
Silt and clay Silt and clay Shelly mud
+1.85 to 1.40 +1.40 to +1.10 +0.50 to 0.40
main channel; and (2) estimate tidal storage volumes for comparison with other mangrove creek systems. In the upper part of the catchment, eight survey transects were made across the most topographically variable and expansive high-intertidal areas (Fig. 2). Elevations were reduced to AHD by surveying into survey bench marker C017 (Grid Reference 05 02 304, 78 65 278 on Australian Mapping Grid, 1984). For the lower part of the catchment, we referred to 1 : 1500 and 1 : 2500 topographic maps of the lower creek locality and adjacent salt flat of Cocoa Creek (contour interval¼ 0.1 m; AUSLIG, 1994a,b).
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Bathymetric profiles of the main channel were recorded to provide cross-sectional data at the main instrument sites for calculation of tidal discharge and sediment transport flux. Geophysical surveying of the main channel was undertaken in August 1994 and June 1998 using a Furuno FG-200 Mark 3 echo-sounder, deployed from a dinghy travelling at speeds of 1 kn or less. Surveying included 16 separate axial sections of the creek and 37 cross-channel transects. All positioning was undertaken using a hand-held global positioning system (GPS-Garmin), aerial photograph (1 : 25 000) and topographic map (1 : 100 000). At the time, the horizontal error of the GPS in the Townsville area was 50 m, thus requiring the use of all three tools for positioning. During the recording of longitudinal bathymetric profiles, reference points were noted every 50–200 m. 2.2. Hydrodynamic and turbidity data This paper draws on a series of hydrodynamic and turbidity datasets, taken within a 4-year period, and totalling over 1.2 instrument years. During this period, wet seasons were extremely dry, with average annual rainfall significantly lower than the long-term average. Individual datasets cover spring–neap cycles of up to 4 weeks in duration, at up to eight sites between the lowintertidal mudflat and the supratidal salt flat, and including most times of the year (the main monitoring sites are shown in Fig. 2 and described in Table 2). Typically, the number and combination of instruments deployed at each monitoring site (as well as the number of monitoring sites) was limited by instrument availability, and is detailed below. Tidal currents and water depths were measured using three types of current meters (Interocean S4, Aanderaa and Seapac 2100). The limited number of current meters and their availability meant that a field deployment could consist of a maximum of three channel sites where tidal currents were measured simultaneously (i.e. containing one instrument in the water column at each site). Most current meter data was recorded at the upper and lower creek sites, close to road access, and with one instrument in the vertical. A site was also located at the mouth, acting as a control point for measuring the import and export of water and sediment. Pressure sensors on the instruments were located at 0.2–1.0 m above the bed, mounted on a frame and placed in the deepest part of the channel at each site. Most frequently, a 1-min average was recorded every 10 min, over deployments of 3–4 weeks duration. Turbidity data were measured using optical backscattering nephelometers (instruments described in Ridd & Larcombe, 1994) attached to a frame in the deepest part of the channel, with sensors located at 0.2 m above the bed. Data were also taken on the salt flat and low-intertidal mudflat, with optical sensors approximately
0.1 m above the bed. A maximum of five nephelometers were deployed simultaneously to obtain the greatest spatial coverage of data (i.e. with one instrument in the vertical). Data are 10-s to 1-min averages recorded every 2–10 min. Nephelometers were cross-calibrated using laboratory standards. Calibrations to mg l1 were undertaken by collecting a minimum of 20 water samples in the field next to a recording nephelometer, or by sampling suspensions of Cocoa Creek sediments and salt water in a laboratory tank, followed by filtering of the water samples (e.g. as described in Larcombe, Ridd, Wilson, & Prytz, 1995). The positioning of all main channel instrument deployments was measured from a reference point on the adjacent bank.
3. Results 3.1. Tidal asymmetry in Cocoa Creek Some mangrove creek systems, including Cocoa Creek, have a characteristic double peak in flow speed during an overbank flood tide (Fig. 3). The tidal asymmetry is therefore slightly different to that described for most shallow estuaries in previous studies (e.g. Boon & Byrne, 1981; Friedrichs & Aubrey, 1988; Speer & Aubrey, 1985). Past usage of the terms flood and ebb dominance in reference to estuarine tidal asymmetry has usually implied that the flood (or ebb) tide is shorter in duration and faster, with a single peak in speed. This inference would be inapplicable here, and so the terms will not be used. A closer examination of the nature of tidal asymmetry of overbank tides in Cocoa Creek, reveals the significantly different consequences for sediment transport in this system. For example, in reference to Fig. 3a and tides of the 24 February 1994, the overbank flood tide has two peaks at around 0.68 m s1 and corresponds to a tidal range of 2.85 m. The subsequent ebb tide has a single peak of 0.71 m s1, (i.e. slightly larger than the flood peaks), but this peak is very short in duration. In addition, the ebb tide range is 2.60 m, slightly smaller than the flood tide. So, although the ebb tide has a higher peak current speed, it is unlikely to be as significant as the flood tide in terms of sediment transport because the flood tide has persistently higher current speeds due to the larger tidal range. 3.1.1. Velocity asymmetry over an annual timescale At the mouth, lower creek and upper creek sites during neap and intermediate tides, maximum tidal current speeds are usually fastest on the larger flood tide of the day. Flood and ebb speeds generally increase with the onset of a spring tide period (Fig. 3). In the lower creek, maximum speeds recorded in the channel on overbank tides tend to be faster on the flood tide during
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Fig. 2. (a) Locality map of the main instrument sites and surveyed transects at Cocoa Creek. (b) Channel cross-sections at the three main instrument sites. L and R stands for true left and true right (i.e. looking seawards). (c) Topographic profiles of two salt flat areas in the Cocoa Creek catchment.
Table 2 Geomorphological description of the main instrument sites at Cocoa Creek Site
Thalweg distance upstream (km)
Width at MSL (m)
Maximum depth (m AHD)
Channel cross-section (bankfull m2)
Channel shape (in profile)
Bed sediment
Upper creek Mid-creek Lower creek Salt flat Mouth Low-intertidal mudflat
7.5 6.2 1.5 1.5 0.05 0.3
15 21 27 – 30 –
2.0 1.5 5.5 þ1.5 4.0 0.4
40 45 95 – 110 –
Symmetrical Asymmetrical Slightly asymmetrical – Slightly asymmetrical –
Muddy gravel Sand Coarse sand Silt and clay Shelly sandy mud Muddy sand
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Fig. 3. (a) Current speed, direction and tidal elevation (all measured at 1 m above the bed) at the lower creek site for days 18–26 February 1994 (neap to spring tide). Note that tidal speeds on the overbank flood tide are slightly faster. F, larger flood tide of the day; E, larger ebb tide of the day. (b) Current speed (measured at 1 m above the bed) at the lower creek site and current speed) and tidal elevation (both measured at 1 m above the bed) at the upper creek site for days 10–14 December 1993. Note that at the lower creek site, tidal speeds on the overbank ebb tide are fastest. F, larger flood tide of the day; E, larger ebb tide of the day.
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the months of August, September and (early) February and on the ebb tide during July, December, January and (late) February. Here, when the ebb speeds in the channel are fastest, the overbank ebb tide is usually of greater duration (by up to 90 min) than the flood. In the upper creek, tidal velocities were recorded during three spring–neap periods (December and February) when, during overbank tides, the ebb tides were faster than the flood. These datasets show that currents at the upper creek site are strongly asymmetric (Fig. 3). For the months of March–June and October– November, we only have time-series data of <7 days in duration and cannot assess in detail the nature of the tide. However, tidal predictions for Townsville Port for 1995 and 1996 (Queensland Department of Transport, 1993–1996) show two periods of approximately 10.5weeks duration (September–November 1995 and October–December 1996) where only one high water would have exceeded overbank in Cocoa Creek. Thus, based on our observations of Ôwithin-channelÕ tides, we infer that most of these tides may have had a faster flood flow.
3.1.2.3. Overbank flood peak in speed (F2). At sites within the creek, the rapid increase in speed to a second flood peak (F2), occurs when the tide locally is at þ1.6 m AHD and has overbanked to inundate a large area of salt flats (Fig. 2c). F2 maximum speeds are generally slower than F1 (i.e. for high water—HW <þ1.8 m AHD), and occur approximately 70 min before high water.
3.1.2. Tidal stage and velocity–asymmetry relationships The shape of velocity-stage profiles in Cocoa Creek (Fig. 4) can be divided into two types, representing tides which either remain Ôwithin-channelÕ (mostly neap and intermediate tides), or which ÔoverbankÕ (some spring tides).
3.1.3. Water surface slopes Water surface slopes drive tidal current speeds within the creek system and we have investigated the relationships between the timing of maximum water surface slopes and specific tidal velocity peaks. Using depth measurements recorded by pressure sensors on a current meter placed at each site, water surface slopes were calculated between the mouth and the upper creek localities for a small spring tide (HW of þ1.54 m AHD) and a very large spring tide (HW þ 2.22 m AHD). For the flood tide, the steepest surface water slopes were 1.3 104 and 2.7 104 on the small and large overbank tide, respectively, coincident with the within-channel flood peak in speed (F1) recorded at the mouth. Slopes reduced towards bankfull stage (FMIN), to peak again just after the tide overbanked (coincident with the velocity peak F2). On the ebb tide, maximum water surface slopes of 5.4 105 persisted for a 2-h period for the smaller spring tide and slopes up to 2.1 104 occurred for a 6-h period for the larger tide. Calculated total slope errors (i.e. taking into account the accuracy of instruments and differences in water elevation between sites at HW) are 2 105 and 8 105 for the small and large overbank tide, respectively.
(a) For within-channel tides, the fastest speed occurs early in the flood tide (e.g. see also Bayliss-Smith et al., 1979; Woodroffe, 1985b) attaining 0.4–0.5 m s1. Ebb tides do not exceed 0.3 m s1. (b) Overbank tides are characterized by a double peak in speed on the flood tide (Figs. 3 and 4) and an extended peak in speed on the ebb (Fig. 4; see also Larcombe & Ridd, 1996; Lessa & Masselink, 1995; Woodroffe, 1985b). An overbank tide can be divided up into four stages: 1. 2. 3. 4.
Within-channel flood peak in speed (F1); Bankfull minimum flood speed (FMIN); Overbank flood peak in speed (F2); and Within-channel maximum ebb speed (EMAX).
3.1.2.1. Within-channel flood peak in speed (F1). At the mouth, the initial flood peak in speed (F1) may occur within a large range of tidal elevations. At the lower creek site, it occurs mid-tide, approximately at þ0.1 m AHD (i.e. MSL). At the upper creek site the peak is shorter in duration, and occurs when the tide locally is at 0.5 to 0.1 m AHD. Flood tide speeds can reach up to 1.0 m s1. 3.1.2.2. Bankfull minimum speed (FMIN). At all sites, tidal speeds decrease as bankfull height is approached.
3.1.2.4. Within-channel maximum ebb speed (EMAX). At the upper creek site, maximum ebb speeds occur when the ebb tide is between þ1.30 and þ0.95 m AHD (Fig. 4c) whereas further seawards, this stage forms part of a longer extended ebb period (Fig. 4a, b). It should be noted that these four stages have been described for a spring tide period where speeds are fastest on the ebb tide. For overbank tides where speeds are fastest on the flood, the magnitudes of the peaks are different but the tidal stage at which each occurs is the same.
3.1.4. Water budget At three of the main instrument sites, water discharge has been calculated using the velocity area method and a single velocity measurement in the channel (most commonly recorded at 1 m above the bed and subsequently depth-averaged; Fig. 5). Although the presented results are not directly comparable between sites, they indicate the general magnitudes of discharge
Fig. 4. Velocity-stage diagrams at three channel sites (during three spring tide periods where ebb speeds were fastest). The mouth profiles are part of a small spring tide period where the maximum tidal height reached is þ1.56 m AHD. In contrast, the lower creek and upper creek profiles are parts of larger spring tide periods, reaching maximum tidal heights of þ1.85 m AHD and þ1.92 m AHD, respectively. Abbreviations of velocity peaks: F1, initial flood peak; FMIN, bankfull minimum speed; F2, second flood peak; EMAX, within-channel maximum ebb speed. The jagged nature of the profiles at the lower creek locality reflects a higher sampling frequency than used at the upper creek and mouth localities.
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Fig. 5. Tidal discharge curves for within-channel and overbank tidal cycles at the upper creek, lower creek and mouth localities. Water fluxes were estimated using the depth-averaged velocity multiplied by the cross-sectional area of the channel for a given stage when overbank speeds were faster on the ebb tide. Maximum tidal elevations in AHD are shown for each tidal cycle. F, flood tide; E, ebb tide and OB, overbank tide.
present in the main channel of Cocoa Creek. The greatest discharges are in the lower creek and on the ebb tide, and the duration of the ebb tide is longer than the flood at the mouth and lower creek. Using the same datasets water budgets have also been calculated where the total discharge over a tide was determined by summing estimates of discharge for 10-min periods (upper creek and mouth) and 2-min periods (lower creek). At the upper creek, results lie between an export of 3.9 104 m3 and import of 6.8 104 m3, and generally show a slight export, on average 3% of the total combined flood and ebb flux. At the lower creek, results lie between an export of 1.7 105 to 2.9 105 m3, and generally show an average export of 30% of the total combined flood and ebb flux. At the mouth, results lie between an export of 2.4 104 to 1.5 105 m3, and also show an average export of 20% of the total combined flood and ebb flux. Some differences in volume can be
accounted for through the errors involved in using a single point measurement of current to predict the velocity profile across the channel (approximately 12%, Boon, 1978) and the possibility of water exchange at high tide with the adjacent Crocodile Creek catchment (locality shown in Fig. 1). 3.2. Suspended sediment concentrations in the channel 3.2.1. Variation in suspended sediment concentrations between sites The calculated tidal excursion of large overbank tides is >9 km, so at these times, suspended sediment is potentially transported along most of the channel length. During neap and intermediate tides, suspended sediment concentrations (SSCs) throughout the creek generally remain below 100 mg l1 (Fig. 6). On overbank tides, SSCs increase significantly at all sites and a
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Fig. 6. (a) Plot showing the variation in SSCs (at 0.2 m above the bed) over a neap to spring tide period in January 1996 at the mouth, lower creek, mid-creek and upper creek localities. Numbers in brackets denote distance landwards of the mouth. Note the difference in scale on the upper creek dataset as compared with the other three sites. (b) Expanded view of four tides. Note the phase lag between the mouth and upper creek site.
complex distribution of SSCs develops along the creek. On a typical overbank tide: 1. SSCs peak sharply just after the within-channel flood peak in speed (F1) at the mouth, lower creek
(Fig. 6) and upper creek sites. In the mid-creek, SSCs steadily rise (Fig. 6); 2. As bankfull elevation is approached, SSCs decrease rapidly to a minimum at the mouth and lower creek, but decrease less sharply elsewhere (Fig. 6b);
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3. Shortly following the overbank flood peak in speed (F2) at the mouth and lower creek (Fig. 7) sites, a small SSC peak occurs, also present in the mid- and upper creek sites as a small rise in SSCs from an already turbid water column (Fig. 6). SSCs remain high in the mid- and upper creek until high water; and 4. On the ebb tide at the mouth and lower creek, SSCs rise steadily to reach a maximum at approximately MSL, where they remain until low water (Fig. 6). In the upper and mid-creek, SSCs peak locally between þ1.5 and þ0.6 m AHD, i.e. when the flow is contained within the channel.
3.2.2. Variation in SSCs through a spring tide period SSCs are nearly always highest on the first few overbank tides of a spring tide period (e.g. Fig. 7). For example, SSCs calculated at the lower creek during the ebb tide of the higher HW of the day (HW þ1.90 m AHD) are notably reduced compared to the previous overbank tide (from a HW of þ1.92 m AHD), despite similar peak ebb speeds (>0.85 m s1) and rates of tidal rise and fall (Fig. 7). This phenomenon has consistently been recorded in the lower creek, and is also found elsewhere in the creek (e.g. mouth and lower creek in Fig. 6), being more pronounced when the ebb is the faster spring tide current. This indicates that sediment availability is limited for the latter part of the spring tide period.
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3.2.3. Vertical distribution of SSC Vertical profiles of turbidity were measured at various sites along the main channel on three occasions: a small ebb tide during a large spring tide period (27 April 1994); a large spring tide (16 February 1996); and a neap ebb tide following a cyclone event (11 January 1996). On the first two occasions, which represent periods of no freshwater influence, the vertical distribution of SSC was generally uniform, especially on spring tides. On the third occasion, runoff produced a slightly stratified water column but there was only a 20 mg l1 top–bottom difference in SSC. At this time, near-bed SSCs were always highest (generally the basal 0.5 m). In addition, turbidity profiles measured in three cross-channel sites at the mouth during the early flood of an intermediate tide (8 June 1994), showed a maximum cross-channel difference of only 10 mg l1. 3.2.4. Effect of waves on SSCs An analysis of wind data recorded at Townsville airport and SSCs at the lower creek site (for six turbidity datasets collected from July 1993 to February 1994; data not presented but available in Bryce, 2001) showed no correlation between winds and high SSCs. It is inferred here that an increase in local wave climate is a potential mechanism for re-suspending muddy sediments in the
Fig. 7. Current speed (measured at 1 m above the bed) at the lower creek site and near-bed SSCs (at 0.2 m above the bed) on the low-intertidal mudflat, lower creek and salt flat sites during a spring tide period where ebb tide speeds were fastest (10–16 December 1993). Numbers on current speed figure are the maximum high water elevations reached in m AHD. F, larger flood tide of the day; E, larger ebb tide of the day. Note the significantly smaller SSC scale used for the salt flat data in comparison to the other two sites.
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nearshore zone off Cocoa Creek (as found in other parts of Cleveland Bay—Larcombe et al., 1995). During the periods covered by the wind data, wind direction ranged from NE to SE and speeds rarely rose above 10 kn, thus representing typical background conditions during the wet and dry season, rather than conditions related to meteorological storm events. 3.3. Suspended sediment concentrations on the low-intertidal mudflat and salt flat On the low-intertidal mudflat, peak SSCs during within-channel tides rarely exceeded 150 mg l1. During some of the larger overbank tides, there was little or no turbidity registered on the overbank flood tide whereas SSCs of up to 400 mg l1 were observed on the subsequent ebb tide (Fig. 7). SSCs on the salt flat were low (Fig. 7), with a maximum SSC on the flood tide of up to 100 mg l1 on the largest overbank tides but generally 40–60 mg l1 (from observation of five spring tide periods). SSCs recorded on the salt flat during the ebb tide were generally <50 mg l1. 3.3.1. Suspended sediment fluxes in the channel Suspended sediment fluxes have been calculated at the lower creek site and are the product of the mean cross-sectional current and SSC. The calculations assume that suspended sediments are homogenous across the creek and throughout the water column, assumptions which are consistent with the SSC data described above and similar to the findings of Larcombe and Ridd
(1996) in nearby Gordon Creek (locality Fig. 1). At the lower creek site, the vertical velocity profile was derived using the Karman–Prandtl equation, which assumes a logarithmic vertical profile with the depth-averaged velocity occurring at a height of 0.38 of the flow depth above the bed. A roughness length of 1.0 cm was used, after applying the equation of Lettau (1969) to echosounder records of bedforms on the channel bed. Suspended sediment fluxes were calculated over: (a) a spring tide period in July 1993 (for 1-week period, faster ebb tide); (b) a spring tide period in September 1993 (for 1-week period, faster flood tide; and (c) a 3-week period in February 1994, (Fig. 8; faster flood tide see Fig. 3a). The average calculated error on the suspended sediment flux per tide is 21%, comprising 9% for the natural (random) variation in water volume at the mid-estuary site, and 12% for the cross-calibration of the nephelometers and conversion of turbidity data to SSCs. The average systematic error in calculating water volumes at the lower creek per tidal cycle (i.e. an average export of 30% of the total flood and ebb volume, as previously mentioned) was adjusted for in the calculation of sediment fluxes for each flood tide. For within-channel tides, net suspended sediment fluxes per tidal cycle are mostly landwards (Fig. 8), but on overbank tides, net fluxes are strongly seawards. For a complete spring–neap period, calculated net fluxes may be landwards or seawards—for the three datasets
Fig. 8. Measured net suspended sediment flux per tidal cycle at the lower creek locality (February 1994).
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outlined above, results are approximately 61 t seawards (for July) and approximately 465 and 91 t landwards (September and February, respectively). The estimated gross flux of sediment moved for these same datasets were 1936, 1282 and 3368 t for July, September and February, respectively. Taking into consideration the errors involved in calculating the net fluxes per tidal cycle (error bars shown in Fig. 8), the net movement of suspended sediment over a spring–neap period could be very small in either direction.
4. Discussion 4.1. Factors driving tidal asymmetry Tidal asymmetry in shallow estuaries takes the form of unequal durations and/or unequal magnitudes in the flood and ebb tide. It mostly results from the frictional interaction of the tide with the channel and intertidal areas (Friedrichs & Aubrey, 1988; Speer & Aubrey, 1985). In the literature, the type of tidal asymmetry present in a channel has been expressed using the following parameters: (1) the relative tidal amplitude (ratio of offshore M2 tidal amplitude (a) to channel depth at MSL (h) = a/h) and (2) the relative estuarine intertidal storage (ratio of volume of intertidal storage (Vs) to the channel volume at MSL (Vc) ¼ Vs/Vc). In estuaries where the fastest speeds are during the flood tide, tidal asymmetry is thought to be controlled by the a/h ratio, whereas flow in those estuaries with fastest speeds on the ebb are thought to be controlled by a large relative estuarine intertidal storage. For a/h < 0.2, an estuary would tend to have a fast and short ebb tide, and for a/h > 0.3, it would tend to have a fast and short flood tide (Friedrichs & Aubrey, 1988; Table 3). Ratios of the total tidal amplitude to channel depth at MSL were calculated for the lower creek site at Cocoa Creek over four spring tide periods (Table 3). The total tidal amplitude near the creek mouth was used instead of the M2 amplitude because the tidal wave may have already undergone some distortion on passing across Cleveland Bay (i.e. before it reaches Cocoa Creek). Most values of a/h at lower Cocoa Creek fall into the transitional to flood range of Friedrichs and Aubrey (1988) rather than consistently reflecting the measured tidal asymmetry, indicating that the relative tidal
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amplitude does not alone determine the tidal asymmetry in Cocoa Creek. To evaluate more fully the effect of external tidal forcing upon the creek (i.e. the nature of any offshore tidal asymmetry that may be imposed on the system), an analysis of predicted and measured tidal heights (for Townsville port for the period July 1993 to June 1994) was undertaken, and the ratio of the rate of flood rise to ebb fall for overbank tides calculated. Results fall mostly between 1.0 and 1.4 (Fig. 9), indicating that the regional tidal forcing is dominated by a tide with a relatively fast flood tide. The nature of this tidal forcing might partially explain the apparent offset between the calculated a/h results (mostly transitional to faster flood tide—see Table 3) and the measured asymmetry (i.e. Cocoa Creek demonstrates both faster flood and ebb tides at different times of the year), when compared to a/h results found elsewhere (Friedrichs & Aubrey, 1988). Wolanski, Mazda, and Ridd (1992b) associated faster ebb tidal speeds in mangrove creek systems to a high ratio of Vs/Vc, and for seven mangrove creek systems they noted that ratios of Vs : Vc ranged from 2 to 7 (with one exception of 44). This range is comparable to Cocoa Creek (Table 4). Wolanski et al. (1992b) reported the slopes of the swamp substrate were 1–4 103 (mostly 2–3 103), generally steeper than for Cocoa Creek (Table 4). These data indicate that the intertidal storage capacity of overbank tides at Cocoa Creek should be sufficient to repeatedly induce a relatively fast and brief ebb tide, but this is not always the case (e.g. Fig. 3). A potential explanation for this discrepancy is the regional tidal forcing (see above), but there may also be an additional mechanism. The slope of the ebb tide over the salt flats are often steeper than the salt flat substrate itself (1–2 103) so that areas closest to the creek drain
Table 3 Comparison of the Friedrichs and Aubrey (1988) ratio of relative tidal amplitude (a/h) findings (see text) to calculated a/h values for overbank tides at Cocoa Creek
Friedrichs and Aubrey (1988) Lower Cocoa Creek on overbank tidal (this paper)
Faster ebb tide
Faster flood tide
<0.2 0.21–0.23
>0.3 0.18–0.47
Fig. 9. Calculated ratios of rising rate of flood tide to falling rate of ebb tide for overbank tides at Cocoa Creek over a 12 month period. Data are from measured and predicted tidal heights at Townsville Port where the elevation of high water exceeds þ1.5 m AHD.
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Table 4 Measured substrate slopes and calculated estuarine intertidal storage ratios (Vs : Vc) at significant tidal elevations, Cocoa Creek Tidal elevation (m AHD)
Substrate slope
Intertidal storage volume : channel storage volume (Vs : Vc)
+1.85 (upper limit of salt flat) +1.60 (salt flat) +1.50 (overbank) +1.30 (mangrove fringe)
4.3 104
8.6
1–2 103 3.8 2 104–1 103 2.6 1101 1.7
first (Aucan, 1998). Later in the ebb, the upper salt flat areas slowly drain through small tributaries, resulting in a prolonged ebb flow. The tidal asymmetry of Cocoa Creek is thus dependent on the interaction between at least two controls: the offshore forcing of a shorter and faster flood tide, and the internal forcing of a faster ebb tide, imparted by the intertidal storage effect. This balance is significant because of its consequences for sediment transport. 4.2. Suspended sediment transport 4.2.1. Controls on SSC and suspended sediment fluxes The primary control upon SSCs and sediment transport in Cocoa Creek are the tidal currents. On timescales of hours, advection is a significant factor in the SSC variations, especially during overbank tides (Fig. 10). At most sites, on the flood tide, SSCs rise as mud is resuspended off the bed, but the exception is in mid-creek, where the bed is sandy (Bryce, 2001) and SSCs tend to peak only upon the arrival of turbid water from the lower creek. Later in the flood tide, with deepening water and slowing tidal currents, SSCs in the nearshore zone decrease, so that less turbid water is advected into the creek system. Over timescales of weeks, for example between two spring tide periods, the relative magnitude of SSCs on overbank tides appears to be, in part, controlled by the local availability of re-suspendable sediment (e.g. Fig. 6). Differences in SSC magnitudes do not appear to be due to the effects of wind under background conditions (i.e. through re-suspension of muddy nearshore sediment which is then available for import by tidal currents), although this may become more important during storm events. During all neap and intermediate tides (i.e. withinchannel tides), suspended sediment transport in the channel is controlled by the shorter and faster flood tide, in which suspended sediment fluxes are moved landwards. Greater sediment transport occurs during spring tide periods, where the direction and magnitude of net suspended sediment flux is influenced by: (a) Tidal asymmetry. For spring tide periods with faster flood overbank tides, the net landward movement of
suspended sediment is dependent on the total amount moved during within-channel tides. This is because the maximum current speeds of the overbank flood and ebb tides are of similar magnitude (Fig. 3a) and result in little net flux. In contrast, for spring tide periods with a faster overbank ebb tide, the export of suspended sediment is driven by an ebb tide which is far faster than the preceding flood flow (Fig. 3b). (b) Sediment availability. During neap tides net suspended sediment fluxes per tidal cycle tend to be below 20 t and the remaining within-channel tides rarely exceed 80 t (Fig. 11). Approaching the largest overbank tides (i.e. HW approximately þ1.85 m AHD), large amounts of suspended sediment are exported on the first few overbank tides (immediately prior to the largest tide itself), limiting the amount of suspended sediment available for export on subsequent large tides. This produces a wide variation in net suspended sediment fluxes at equivalent high tidal ranges (Fig. 11) and can be considered as a hysteresis effect in sediment concentration. 4.2.2. Implications for suspended sediment transport on annual and longer timescales During all neap and intermediate tides, suspended sediments are pumped towards the head of the creek, thereby providing a temporary local supply of sediment for potential transport on the first few overbank tides, some of which may be removed by overbank tides. SSC data from the salt flat (Fig. 7) suggests that, during overbank flood tides, some finegrained sediment is transported up onto the salt flat, while some may also be transported seawards on the subsequent ebb. In the latter part of the year (late September to early December) the relative absence of overbank tides indicates a sustained period of sediment accumulation in the head of the creek. Later (December–February), when the wet season coincides with spring tide periods of mostly faster ebb currents and seaward-directed net suspended sediment fluxes, this store of sediment might be exported seawards out of the creek. SSC data from the low-intertidal mudflat at the creek mouth suggests that on the ebb tide, suspended sediment is exported from the system (Fig. 7). We do not have reliable sediment transport data for the months of March–June, but we infer from the tidal characteristics that there will be periods of flood-directed and ebb-directed net suspended sediment flux. It is thus clear that the direction of net suspended sediment fluxes in Cocoa Creek is highly variable on timescales of months. On the timescale of years to decades, despite the large nature of our datasets it is not obvious as to whether Cocoa Creek is either a net importer or exporter of muddy sediment. SSCs from the salt flat suggest that
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Fig. 10. Plan view of the turbidity distribution (expressed as SSCs) in Cocoa Creek at various stages of an overbank tide on 18 January 1996, together with the main controlling factors. The tidal elevations expressed in AHD at the mouth and upper creek sites are taken from real depth values. The distribution pattern of SSCs is also based on turbidity data measured during a deployment of four nephelometers along the main channel (locations shown in Fig. 2a), each placed at approximately 0.2 m above the bed.
sediment is not being deposited on the salt flat, and therefore over geological time, the area in which marine water is held up at high tide, is not being reduced. This implies that large overbank ebb tides will still continue to scour the main channel and remove fine sediment that has been pasted on the walls of the channel during neap and intermediate tides, and including spring tide periods with faster flood tides. In addition, while land-based sources of muddy sediment to the Cocoa Creek tidal system are negligible, the shallow inner shelf of adjacent Cleveland Bay contains a virtually unlimited supply of mobile muddy sediments available to be transported into the small Cocoa Creek system (Carter, Johnson, &
Hooper, 1993). Geologically, the creek system is therefore not limited by sediment supply, and we conclude tentatively that the geomorphology of Cocoa Creek may be near a long-term equilibrium.
5. Conclusions The tidal asymmetry of Cocoa Creek is controlled by the interaction between offshore tidal forcing and the intertidal storage effect of the mangrove swamps and salt flats. The result is that there tends to be a predominance of either faster flood or ebb current speeds
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Fig. 11. Measured net suspended sediment flux per tidal cycle at the lower creek site of Cocoa Creek versus measured high-water elevation at Townsville Port (the closest port datum). Note that most seaward-directed suspended sediment fluxes occur on overbank tides. We have illustrated fluxes calculated for successive overbank tides of late February 1994 (spring period commencing in Fig. 3a). The data describe an open loop whereby tides of equivalent elevation (most notably where HW > þ1.85 m AHD) produce high net seaward suspended sediment fluxes early in the spring tide period, but up to 100 t less only a few tides later. m July 1993; } September 1993; d February 1994.
during certain periods of the year and that the nature of tidal asymmetry is therefore not the same for all overbank tides. For within-channel tides, a small amount of suspended sediment is transported mostly landwards, controlled by tidal asymmetry. Significant tidal suspended sediment transport in the channel is only initiated at overbank height (approximately þ1.5 m AHD) when resulting net fluxes are mostly seawards. On the larger overbank tides (where the maximum tidal height > þ1.85 m AHD) net sediment fluxes are reduced because of a limited supply of available material. The complex but strong relationships between hydrodynamics, sediment transport and geomorphology demonstrated in this and some other tropical tidedominated systems (e.g. Bryce et al., 1998; Larcombe & Ridd, 1995, 1996) indicates that complex sediment transport regimes are likely to be the rule rather than the exception. Given this complexity, and that the availability of sediment within these systems is limited at times, sediment fluxes should not be estimated for such systems using only sediment transport equations. This study also highlights the importance and necessity of using long time-series datasets to unravel linkages between sedimentary processes and the long-term sedimentary status of tidal mangrove creek systems.
Acknowledgements Many thanks go to Ross Hyne for assistance with fieldwork, and loan of a boat, vehicles and other field
equipment. Thanks are also due to Mick Fitzpatrick from the Townsville Port Authority for access to tidal data, Russell Jaycock from the Bureau of Meteorology for the wind data and Gregg Brunskill of the Australian Institute of Marine Science for access to AUSLIG topographic maps. This work was funded by an ARC Large Grant to P.L. and P.V.R., and by James Cook University. The two anonymous journal reviewers are thanked for their helpful comments in improving the manuscript.
References Aucan, J. (1998). Hydrodynamic model of the circulation in a mangrove fringed tidal creek (50 pp.). Unpublished postgraduate diploma thesis, James Cook University, Townsville. Australian Land Information Group-AUSLIG (1994). 1 : 1500 Topographic map Cocoa Creek Townsville, NI5-359 (B1). Australian Land Information Group-AUSLIG (1994). 1 : 2500 Topographic map Cocoa Creek Townsville, NI5-358 (B1). Bayliss-Smith, T. P., Healy, R., Lailey, R., Spencer, T., & Stoddart, D. R. (1979). Tidal flows in salt marsh creeks. Estuarine and Coastal Marine Science 9, 235–255. Boon, J. D. (1975). Tidal discharge asymmetry in a salt marsh drainage system. Limnology and Oceanography 20, 71–80. Boon, J. D. (1978). Suspended-solids transport in a saltmarsh creek—an analysis of errors. In B. Kjerfve (Ed.), Estuarine transport processes (pp. 147–159). Columbia, SC: University of South Carolina Press. Boon, J. D., & Byrne, R. J. (1981). On basin hypsometry and the morphodynamic response of coastal inlet systems. Marine Geology 40, 27–48. Bryce, S. (2001). Controlling factors on sediment transport in mangrove creek systems of North-East Queensland (156 pp.). Unpublished PhD thesis, James Cook University, Townsville.
S. Bryce et al. / Estuarine, Coastal and Shelf Science 56 (2003) 415–431 Bryce, S., Larcombe, P., & Ridd, P. V. (1998). The relative importance of landward-directed tidal sediment transport versus freshwater flood events in the Normanby River estuary, Cape York Peninsula, Australia. Marine Geology 149, 55–78. Carter, R. M., Johnson, D. P., & Hooper, K. G. (1993). Episodic postglacial sea-level rise and the sedimentary evolution of a tropical embayment (Cleveland Bay, Great Barrier Reef shelf, Australia). Australian Journal of Earth Science 40, 229–255. Fisher, P. R. (1994). Characteristic circulation and sedimentation in the Rufiji Delta, Tanzania. Frontier-Tanzania technical report no. 13 (92 pp.): The Society for Environmental Exploration, UK. Friedrichs, C. T., & Aubrey, D. G. (1988). Non-linear tidal distortion in shallow well-mixed estuaries: a synthesis. Estuarine, Coastal and Shelf Science 27, 521–545. Furukawa, K., Wolanski, E., & Mueller, H. R. (1997). Currents and sediment transport in mangrove forests. Estuarine, Coastal and Shelf Science 44, 301–310. Larcombe, P., & Ridd, P. V. (1995). Megaripple dynamics and sediment transport in a mesotidal mangrove creek: implications for palaeoflow reconstructions. Sedimentology 42, 593–606. Larcombe, P., & Ridd, P. V. (1996). Dry season hydrodynamics and sediment transport in a mangrove creek. In C. Pattariatchi (Ed.), Mixing processes in estuaries and coastal seas, Coastal and estuarine studies 46 (pp. 388–404). Washington, DC: American Geophysical Union. Larcombe, P., Ridd, P. V., Wilson, B., & Prytz, A. (1995). Factors controlling suspended sediment on inner-shelf coral reefs, Townsville, Australia. Coral Reefs 14, 163–171. Lessa, G. (1995). Morphological evolution of macrotidal estuarine channels—central Queensland, Australia. The European Geographer 9, 152–171. Lessa, G., & Masselink, G. (1995). Morphodynamic evolution of a macrotidal barrier estuary. Marine Geology 129, 25–46.
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Lettau, H. (1969). Note on aerodynamic roughness-parameter estimation on the basis of roughness-element descriptions. Journal of Applied Meteorology 8, 828–832. Pethick, J. S. (1980). Velocity surges and asymmetry in tidal channels. Estuarine and Coastal Marine Science 11, 331–345. Queensland Department of Transport (1993–1996). Queensland tide tables. Brisbane: Queensland Government. Ridd, P. V., & Larcombe, P. (1994). Biofouling controls and optical backscatter suspended sediment sensors. Marine Geology 116, 255–258. Ridd, P., Renagi, S., Hollins, S., & Brunskill, G. (1997). Water, salt and nutrient fluxes of tropical tidal salt flats. Mangroves and Salt Marshes 1, 229–238. Speer, P. E., & Aubrey, D. G. (1985). A study of non-linear tidal propagation in shallow inlet/estuarine systems. Part II: Theory. Estuarine, Coastal and Shelf Science 21, 207–224. Wolanski, E., Gibbs, R. J., Mazda, Y., Mehta, A., & King, B. (1992). The role of turbulence in the settling of mud flocs. Journal of Coastal Research 8, 35–46. Wolanski, E., Jones, M., & Bunt, J. S. (1980). Hydrodynamics of a tidal creek mangrove swamp system. Australian Journal of Marine and Freshwater Research 31, 431–450. Wolanski, E., Mazda, Y., & Ridd, P. (1992). Mangrove Hydrodynamics. In A. I. Robertson, & D. M. Alongi (Eds.), Tropical mangrove ecosystems (pp. 43–62). Washington, DC: American Geophysical Union. Woodroffe, C. D. (1985). Studies of a mangrove basin, Tuff Crater, New Zealand: III. The flux of organic and inorganic particulate matter. Estuarine, Coastal and Shelf Science 20, 447–461. Woodroffe, C. D. (1985). Studies of a mangrove basin, Tuff Crater, New Zealand: II. Comparison of volumetric and velocity-area methods of estimating tidal flux. Estuarine, Coastal and Shelf Science 20, 431–445.
Hydrodynamic and geomorphological controls on suspended sediment transport in mangrove creek systems, a case study: Cocoa Creek, Townsville, Australia S. Brycea,*, P. Larcombea, P.V. Riddb a Marine Geophysical Laboratory, School of Earth Sciences, James Cook University, Townsville 4811, Australia Marine Geophysical Laboratory, School of Computer Science, Mathematics and Physics, James Cook University, Townsville 4811, Australia
b
Received 15 February 1999; received in revised form 8 July 1999; accepted 25 February 2002
Abstract In tide-dominated sedimentary systems, close relationships exist between tidal hydrodynamics, sediment transport and geomorphology. Tropical coastlines contain many tide-dominated mangrove creeks, yet few studies to date have examined the detail of such relationships for these environments. Time-series observations of tidal height, currents and suspended sediment concentrations were taken between 1992 and 1996 in Cocoa Creek, a mangrove creek system near Townsville, NE Australia. The creek and surrounding mangrove swamps and salt flats were surveyed with an echo-sounder and total survey station, respectively. For Ôwithin-channelÕ tides, the flood tide is always the fastest, at up to 0.5 m s1. In contrast, for overbank tides (i.e. tidal height > þ1.5 m Australian Height Datum, AHD) ebb currents are fastest in July, December and January, but flood currents are fastest in August and September, at up to 1 m s1 in both cases. The tidal asymmetry of overbank tides in Cocoa Creek is controlled by the interaction between offshore tidal forcing and the intertidal storage effect of the mangrove swamps and salt flats, with the result being that during certain periods of the year there tends to be a predominance of either faster flood or ebb velocities on overbank tides. Significant tidal suspended sediment transport in the channel is only initiated at overbank height. On overbank tides, measured net suspended sediment fluxes in the channel are mostly seaward-directed (up to 180 t per tidal cycle). However, the net flux measured over a neap–spring period may be either landwards or seawards (up to 465 and 60 t, respectively). Furthermore, on the larger overbank tides (where the maximum tidal height > þ1.85 m AHD) net sediment fluxes may be reduced because of a limited supply of available material. Thus hydrodynamic and sediment sampling durations of up to a month may not be representative of long-term trends. Given that our large dataset has not identified a clear long-term net transport direction within the creek system, we conclude tentatively that the geomorphology of Cocoa Creek may be near a long-term equilibrium. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: tidal creek; mangroves; sediment transport; Australia
1. Introduction Studies of hydrodynamics and sediment transport in mostly temperate salt marsh systems (Bayliss-Smith, Healy, Lailey, Spencer, & Stoddart, 1979; Boon, 1975; Boon & Byrne, 1981; Friedrichs & Aubrey, 1988; Pethick, 1980) have provided the initial understanding of hydrodynamic and geomorphological controls on sediment transport for tide-dominated creeks and
* Corresponding author. E-mail address: [email protected] (S. Bryce).
estuaries. Most of these studies used tidal height data to model tidal hydrodynamics, and sediment transport equations to estimate sediment fluxes and the net direction of sediment movement. Some of this work formed the basis for early attempts to understand sediment transport processes, and calculate sediment fluxes and transport rates in their tropical equivalents, mangrove creek systems (Fisher, 1994; Lessa, 1995; Lessa & Masselink, 1995; Wolanski, Jones, & Bunt, 1980; Woodroffe, 1985a). These studies employed methods such as water sampling, applying sediment transport equations and computer modelling, and they commonly assumed that sediment transport patterns
0272-7714/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0272-7714(02)00192-0
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reflected tidal asymmetry, and, further, that tidal asymmetry is similar for each spring tide period. Using these assumptions some relationships between tidal dynamics, sediment transport and geomorphology in mangrove creek systems have thus been proposed (Lessa & Masselink, 1995; Wolanski et al., 1980; Woodroffe, 1985a,b). In tidal mangrove creeks, Wolanski et al. (1980) found that on overbank tides, large water surface gradients are created due to large frictional effects caused by mangrove vegetation density, producing a shorter and faster ebb tide that helps to scour the bed of the main creek channel and thus export sediments from the creek system. Lessa and Masselink (1995) and Woodroffe (1985b) also found a similar velocity asymmetry relationship in the channel with peak flood and ebb speeds increasing as the maximum tidal elevation is increased. The magnitude of suspended sediment fluxes was found to be dependent on the maximum tidal elevation reached (Woodroffe, 1985a). However, studies which have used time-series data to actually measure sediment transport (e.g. Bryce, Larcombe, & Ridd, 1998; Furukawa, Wolanski, & Mueller, 1997; Larcombe & Ridd, 1995, 1996; Wolanski, Gibbs, Mazda, Mehta, & King, 1992a) have revealed a high degree of spatial and temporal complexity in the sediment dynamics of these tidal systems. While findings to date indicate that mangrove swamps are most likely to be places of sediment accumulation (Furukawa et al., 1997; Larcombe & Ridd, 1996), the sediment dynamics within the main channel remain unclear, and as a consequence, it is not known whether these tidal creek systems are net importers or exporters of sediment (Larcombe & Ridd, 1995, 1996; Woodroffe, 1985a). No long-term datasets of measured currents and sediment transport have been presented for mangrove creek systems. This paper details the tidal hydrodynamics, main geomorphic features and sediment transport processes of Cocoa Creek, a small mangrove creek system near Townsville, Australia. The datasets were collected over a period of 4 years and cover different temporal and spatial scales that are representative of long-term trends. Firstly, we investigate the internal and external factors responsible for the tidal asymmetry of the system, and secondly assess their effect upon suspended sediment transport as well as noting issues of local sediment availability. 1.1. Cocoa Creek Cocoa Creek is one of three tidal mangrove creek systems in the southern part of Cleveland Bay near Townsville, north Queensland (Fig. 1). The main channel of Cocoa Creek meanders for 9.5 km through a chenier plain, and extends 1.2 km seawards of the mouth across low-intertidal mudflats. The channel is fringed by a 30–150 m-wide mangrove swamp with the
largest trees (<7 m high) located at the mouth. The total catchment area is approximately 21 km2 of which 70% is made up of salt flats and mangrove swamps. There is a distinct vertical zonation of major geomorphological units in the catchment (Table 1). Annual rainfall at Townsville is approximately 1100 mm year1 of which 65% falls in the summer wet season (January–March), so that freshwater input is minimal to the creek for much of the year. The effects of freshwater runoff events during the wet season are very limited because only around 30% of the catchment sits above the supratidal zone and any significant rates of rainfall (even up to 30 mm h1) will be rapidly mixed into the strongly tidal system. During the wet season, winds are inconsistent northerlies of 5–7 kn and may only reach significantly greater speeds and variable direction when associated with the passing of tropical cyclones. The dry season (May–November) experiences consistent southeasterly winds of 10–15 kn. Tides are semi-diurnal and mesotidal with mean spring and neap tidal ranges of 2.3 and 0.4 m, respectively. Marine water initially enters the mangroves and salt flats via a network of small tributaries before overbanking of the main channel takes place at a tidal elevation of approximately þ1.5 m Australian Height Datum (AHD). The maximum tidal height reached in the Townsville area is þ2.1 m AHD. Mean sea level here is 0.1 m above AHD. At Cocoa Creek, only some spring tide periods include overbank tides, and periods of 3.5-week duration between overbank tides are common. Strong tidal currents ensure that the water column is vertically well-mixed at all times of the year. Groundwater outflow to the main channel is thought to be minimal because the surrounding salt flat stratigraphy is composed of mostly impermeable silts and clays (Ridd, Renagi, Hollins, & Brunskill, 1997).
2. Data collection and analysis Many coastal plain settings around the world, including most of northern Australia, contain extensive wetland/intertidal systems with often poorly defined catchment boundaries. Such systems may act as open drainage systems, that is water may enter and leave the system in a number of places (Fig. 1). Spatial coverage of data therefore becomes important when monitoring the movement of water and sediment within a specific creek system. 2.1. Topographic and bathymetric data Topographic surveys of the high-intertidal areas of Cocoa Creek were made to: (1) investigate relationships between the main geomorphic features and the timing of measured peaks in tidal current velocity at sites in the
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Fig. 1. Locality map and geomorphology of Cocoa Creek and the adjacent coastal plain.
Table 1 Major geomorphological features within the intertidal area of Cocoa Creek catchment and the corresponding tidal elevation range at which they occur Geomorphology Chenier/beach ridges Salt water couch meadow Salt flat Mangrove swamp Low-intertidal mudflats
Substrate lithology
Elevation range (m AHD)
Sand and gravel Clay
+3.65 to +2.00 +2.00 to +1.85
Silt and clay Silt and clay Shelly mud
+1.85 to 1.40 +1.40 to +1.10 +0.50 to 0.40
main channel; and (2) estimate tidal storage volumes for comparison with other mangrove creek systems. In the upper part of the catchment, eight survey transects were made across the most topographically variable and expansive high-intertidal areas (Fig. 2). Elevations were reduced to AHD by surveying into survey bench marker C017 (Grid Reference 05 02 304, 78 65 278 on Australian Mapping Grid, 1984). For the lower part of the catchment, we referred to 1 : 1500 and 1 : 2500 topographic maps of the lower creek locality and adjacent salt flat of Cocoa Creek (contour interval¼ 0.1 m; AUSLIG, 1994a,b).
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Bathymetric profiles of the main channel were recorded to provide cross-sectional data at the main instrument sites for calculation of tidal discharge and sediment transport flux. Geophysical surveying of the main channel was undertaken in August 1994 and June 1998 using a Furuno FG-200 Mark 3 echo-sounder, deployed from a dinghy travelling at speeds of 1 kn or less. Surveying included 16 separate axial sections of the creek and 37 cross-channel transects. All positioning was undertaken using a hand-held global positioning system (GPS-Garmin), aerial photograph (1 : 25 000) and topographic map (1 : 100 000). At the time, the horizontal error of the GPS in the Townsville area was 50 m, thus requiring the use of all three tools for positioning. During the recording of longitudinal bathymetric profiles, reference points were noted every 50–200 m. 2.2. Hydrodynamic and turbidity data This paper draws on a series of hydrodynamic and turbidity datasets, taken within a 4-year period, and totalling over 1.2 instrument years. During this period, wet seasons were extremely dry, with average annual rainfall significantly lower than the long-term average. Individual datasets cover spring–neap cycles of up to 4 weeks in duration, at up to eight sites between the lowintertidal mudflat and the supratidal salt flat, and including most times of the year (the main monitoring sites are shown in Fig. 2 and described in Table 2). Typically, the number and combination of instruments deployed at each monitoring site (as well as the number of monitoring sites) was limited by instrument availability, and is detailed below. Tidal currents and water depths were measured using three types of current meters (Interocean S4, Aanderaa and Seapac 2100). The limited number of current meters and their availability meant that a field deployment could consist of a maximum of three channel sites where tidal currents were measured simultaneously (i.e. containing one instrument in the water column at each site). Most current meter data was recorded at the upper and lower creek sites, close to road access, and with one instrument in the vertical. A site was also located at the mouth, acting as a control point for measuring the import and export of water and sediment. Pressure sensors on the instruments were located at 0.2–1.0 m above the bed, mounted on a frame and placed in the deepest part of the channel at each site. Most frequently, a 1-min average was recorded every 10 min, over deployments of 3–4 weeks duration. Turbidity data were measured using optical backscattering nephelometers (instruments described in Ridd & Larcombe, 1994) attached to a frame in the deepest part of the channel, with sensors located at 0.2 m above the bed. Data were also taken on the salt flat and low-intertidal mudflat, with optical sensors approximately
0.1 m above the bed. A maximum of five nephelometers were deployed simultaneously to obtain the greatest spatial coverage of data (i.e. with one instrument in the vertical). Data are 10-s to 1-min averages recorded every 2–10 min. Nephelometers were cross-calibrated using laboratory standards. Calibrations to mg l1 were undertaken by collecting a minimum of 20 water samples in the field next to a recording nephelometer, or by sampling suspensions of Cocoa Creek sediments and salt water in a laboratory tank, followed by filtering of the water samples (e.g. as described in Larcombe, Ridd, Wilson, & Prytz, 1995). The positioning of all main channel instrument deployments was measured from a reference point on the adjacent bank.
3. Results 3.1. Tidal asymmetry in Cocoa Creek Some mangrove creek systems, including Cocoa Creek, have a characteristic double peak in flow speed during an overbank flood tide (Fig. 3). The tidal asymmetry is therefore slightly different to that described for most shallow estuaries in previous studies (e.g. Boon & Byrne, 1981; Friedrichs & Aubrey, 1988; Speer & Aubrey, 1985). Past usage of the terms flood and ebb dominance in reference to estuarine tidal asymmetry has usually implied that the flood (or ebb) tide is shorter in duration and faster, with a single peak in speed. This inference would be inapplicable here, and so the terms will not be used. A closer examination of the nature of tidal asymmetry of overbank tides in Cocoa Creek, reveals the significantly different consequences for sediment transport in this system. For example, in reference to Fig. 3a and tides of the 24 February 1994, the overbank flood tide has two peaks at around 0.68 m s1 and corresponds to a tidal range of 2.85 m. The subsequent ebb tide has a single peak of 0.71 m s1, (i.e. slightly larger than the flood peaks), but this peak is very short in duration. In addition, the ebb tide range is 2.60 m, slightly smaller than the flood tide. So, although the ebb tide has a higher peak current speed, it is unlikely to be as significant as the flood tide in terms of sediment transport because the flood tide has persistently higher current speeds due to the larger tidal range. 3.1.1. Velocity asymmetry over an annual timescale At the mouth, lower creek and upper creek sites during neap and intermediate tides, maximum tidal current speeds are usually fastest on the larger flood tide of the day. Flood and ebb speeds generally increase with the onset of a spring tide period (Fig. 3). In the lower creek, maximum speeds recorded in the channel on overbank tides tend to be faster on the flood tide during
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Fig. 2. (a) Locality map of the main instrument sites and surveyed transects at Cocoa Creek. (b) Channel cross-sections at the three main instrument sites. L and R stands for true left and true right (i.e. looking seawards). (c) Topographic profiles of two salt flat areas in the Cocoa Creek catchment.
Table 2 Geomorphological description of the main instrument sites at Cocoa Creek Site
Thalweg distance upstream (km)
Width at MSL (m)
Maximum depth (m AHD)
Channel cross-section (bankfull m2)
Channel shape (in profile)
Bed sediment
Upper creek Mid-creek Lower creek Salt flat Mouth Low-intertidal mudflat
7.5 6.2 1.5 1.5 0.05 0.3
15 21 27 – 30 –
2.0 1.5 5.5 þ1.5 4.0 0.4
40 45 95 – 110 –
Symmetrical Asymmetrical Slightly asymmetrical – Slightly asymmetrical –
Muddy gravel Sand Coarse sand Silt and clay Shelly sandy mud Muddy sand
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Fig. 3. (a) Current speed, direction and tidal elevation (all measured at 1 m above the bed) at the lower creek site for days 18–26 February 1994 (neap to spring tide). Note that tidal speeds on the overbank flood tide are slightly faster. F, larger flood tide of the day; E, larger ebb tide of the day. (b) Current speed (measured at 1 m above the bed) at the lower creek site and current speed) and tidal elevation (both measured at 1 m above the bed) at the upper creek site for days 10–14 December 1993. Note that at the lower creek site, tidal speeds on the overbank ebb tide are fastest. F, larger flood tide of the day; E, larger ebb tide of the day.
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the months of August, September and (early) February and on the ebb tide during July, December, January and (late) February. Here, when the ebb speeds in the channel are fastest, the overbank ebb tide is usually of greater duration (by up to 90 min) than the flood. In the upper creek, tidal velocities were recorded during three spring–neap periods (December and February) when, during overbank tides, the ebb tides were faster than the flood. These datasets show that currents at the upper creek site are strongly asymmetric (Fig. 3). For the months of March–June and October– November, we only have time-series data of <7 days in duration and cannot assess in detail the nature of the tide. However, tidal predictions for Townsville Port for 1995 and 1996 (Queensland Department of Transport, 1993–1996) show two periods of approximately 10.5weeks duration (September–November 1995 and October–December 1996) where only one high water would have exceeded overbank in Cocoa Creek. Thus, based on our observations of Ôwithin-channelÕ tides, we infer that most of these tides may have had a faster flood flow.
3.1.2.3. Overbank flood peak in speed (F2). At sites within the creek, the rapid increase in speed to a second flood peak (F2), occurs when the tide locally is at þ1.6 m AHD and has overbanked to inundate a large area of salt flats (Fig. 2c). F2 maximum speeds are generally slower than F1 (i.e. for high water—HW <þ1.8 m AHD), and occur approximately 70 min before high water.
3.1.2. Tidal stage and velocity–asymmetry relationships The shape of velocity-stage profiles in Cocoa Creek (Fig. 4) can be divided into two types, representing tides which either remain Ôwithin-channelÕ (mostly neap and intermediate tides), or which ÔoverbankÕ (some spring tides).
3.1.3. Water surface slopes Water surface slopes drive tidal current speeds within the creek system and we have investigated the relationships between the timing of maximum water surface slopes and specific tidal velocity peaks. Using depth measurements recorded by pressure sensors on a current meter placed at each site, water surface slopes were calculated between the mouth and the upper creek localities for a small spring tide (HW of þ1.54 m AHD) and a very large spring tide (HW þ 2.22 m AHD). For the flood tide, the steepest surface water slopes were 1.3 104 and 2.7 104 on the small and large overbank tide, respectively, coincident with the within-channel flood peak in speed (F1) recorded at the mouth. Slopes reduced towards bankfull stage (FMIN), to peak again just after the tide overbanked (coincident with the velocity peak F2). On the ebb tide, maximum water surface slopes of 5.4 105 persisted for a 2-h period for the smaller spring tide and slopes up to 2.1 104 occurred for a 6-h period for the larger tide. Calculated total slope errors (i.e. taking into account the accuracy of instruments and differences in water elevation between sites at HW) are 2 105 and 8 105 for the small and large overbank tide, respectively.
(a) For within-channel tides, the fastest speed occurs early in the flood tide (e.g. see also Bayliss-Smith et al., 1979; Woodroffe, 1985b) attaining 0.4–0.5 m s1. Ebb tides do not exceed 0.3 m s1. (b) Overbank tides are characterized by a double peak in speed on the flood tide (Figs. 3 and 4) and an extended peak in speed on the ebb (Fig. 4; see also Larcombe & Ridd, 1996; Lessa & Masselink, 1995; Woodroffe, 1985b). An overbank tide can be divided up into four stages: 1. 2. 3. 4.
Within-channel flood peak in speed (F1); Bankfull minimum flood speed (FMIN); Overbank flood peak in speed (F2); and Within-channel maximum ebb speed (EMAX).
3.1.2.1. Within-channel flood peak in speed (F1). At the mouth, the initial flood peak in speed (F1) may occur within a large range of tidal elevations. At the lower creek site, it occurs mid-tide, approximately at þ0.1 m AHD (i.e. MSL). At the upper creek site the peak is shorter in duration, and occurs when the tide locally is at 0.5 to 0.1 m AHD. Flood tide speeds can reach up to 1.0 m s1. 3.1.2.2. Bankfull minimum speed (FMIN). At all sites, tidal speeds decrease as bankfull height is approached.
3.1.2.4. Within-channel maximum ebb speed (EMAX). At the upper creek site, maximum ebb speeds occur when the ebb tide is between þ1.30 and þ0.95 m AHD (Fig. 4c) whereas further seawards, this stage forms part of a longer extended ebb period (Fig. 4a, b). It should be noted that these four stages have been described for a spring tide period where speeds are fastest on the ebb tide. For overbank tides where speeds are fastest on the flood, the magnitudes of the peaks are different but the tidal stage at which each occurs is the same.
3.1.4. Water budget At three of the main instrument sites, water discharge has been calculated using the velocity area method and a single velocity measurement in the channel (most commonly recorded at 1 m above the bed and subsequently depth-averaged; Fig. 5). Although the presented results are not directly comparable between sites, they indicate the general magnitudes of discharge
Fig. 4. Velocity-stage diagrams at three channel sites (during three spring tide periods where ebb speeds were fastest). The mouth profiles are part of a small spring tide period where the maximum tidal height reached is þ1.56 m AHD. In contrast, the lower creek and upper creek profiles are parts of larger spring tide periods, reaching maximum tidal heights of þ1.85 m AHD and þ1.92 m AHD, respectively. Abbreviations of velocity peaks: F1, initial flood peak; FMIN, bankfull minimum speed; F2, second flood peak; EMAX, within-channel maximum ebb speed. The jagged nature of the profiles at the lower creek locality reflects a higher sampling frequency than used at the upper creek and mouth localities.
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Fig. 5. Tidal discharge curves for within-channel and overbank tidal cycles at the upper creek, lower creek and mouth localities. Water fluxes were estimated using the depth-averaged velocity multiplied by the cross-sectional area of the channel for a given stage when overbank speeds were faster on the ebb tide. Maximum tidal elevations in AHD are shown for each tidal cycle. F, flood tide; E, ebb tide and OB, overbank tide.
present in the main channel of Cocoa Creek. The greatest discharges are in the lower creek and on the ebb tide, and the duration of the ebb tide is longer than the flood at the mouth and lower creek. Using the same datasets water budgets have also been calculated where the total discharge over a tide was determined by summing estimates of discharge for 10-min periods (upper creek and mouth) and 2-min periods (lower creek). At the upper creek, results lie between an export of 3.9 104 m3 and import of 6.8 104 m3, and generally show a slight export, on average 3% of the total combined flood and ebb flux. At the lower creek, results lie between an export of 1.7 105 to 2.9 105 m3, and generally show an average export of 30% of the total combined flood and ebb flux. At the mouth, results lie between an export of 2.4 104 to 1.5 105 m3, and also show an average export of 20% of the total combined flood and ebb flux. Some differences in volume can be
accounted for through the errors involved in using a single point measurement of current to predict the velocity profile across the channel (approximately 12%, Boon, 1978) and the possibility of water exchange at high tide with the adjacent Crocodile Creek catchment (locality shown in Fig. 1). 3.2. Suspended sediment concentrations in the channel 3.2.1. Variation in suspended sediment concentrations between sites The calculated tidal excursion of large overbank tides is >9 km, so at these times, suspended sediment is potentially transported along most of the channel length. During neap and intermediate tides, suspended sediment concentrations (SSCs) throughout the creek generally remain below 100 mg l1 (Fig. 6). On overbank tides, SSCs increase significantly at all sites and a
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Fig. 6. (a) Plot showing the variation in SSCs (at 0.2 m above the bed) over a neap to spring tide period in January 1996 at the mouth, lower creek, mid-creek and upper creek localities. Numbers in brackets denote distance landwards of the mouth. Note the difference in scale on the upper creek dataset as compared with the other three sites. (b) Expanded view of four tides. Note the phase lag between the mouth and upper creek site.
complex distribution of SSCs develops along the creek. On a typical overbank tide: 1. SSCs peak sharply just after the within-channel flood peak in speed (F1) at the mouth, lower creek
(Fig. 6) and upper creek sites. In the mid-creek, SSCs steadily rise (Fig. 6); 2. As bankfull elevation is approached, SSCs decrease rapidly to a minimum at the mouth and lower creek, but decrease less sharply elsewhere (Fig. 6b);
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3. Shortly following the overbank flood peak in speed (F2) at the mouth and lower creek (Fig. 7) sites, a small SSC peak occurs, also present in the mid- and upper creek sites as a small rise in SSCs from an already turbid water column (Fig. 6). SSCs remain high in the mid- and upper creek until high water; and 4. On the ebb tide at the mouth and lower creek, SSCs rise steadily to reach a maximum at approximately MSL, where they remain until low water (Fig. 6). In the upper and mid-creek, SSCs peak locally between þ1.5 and þ0.6 m AHD, i.e. when the flow is contained within the channel.
3.2.2. Variation in SSCs through a spring tide period SSCs are nearly always highest on the first few overbank tides of a spring tide period (e.g. Fig. 7). For example, SSCs calculated at the lower creek during the ebb tide of the higher HW of the day (HW þ1.90 m AHD) are notably reduced compared to the previous overbank tide (from a HW of þ1.92 m AHD), despite similar peak ebb speeds (>0.85 m s1) and rates of tidal rise and fall (Fig. 7). This phenomenon has consistently been recorded in the lower creek, and is also found elsewhere in the creek (e.g. mouth and lower creek in Fig. 6), being more pronounced when the ebb is the faster spring tide current. This indicates that sediment availability is limited for the latter part of the spring tide period.
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3.2.3. Vertical distribution of SSC Vertical profiles of turbidity were measured at various sites along the main channel on three occasions: a small ebb tide during a large spring tide period (27 April 1994); a large spring tide (16 February 1996); and a neap ebb tide following a cyclone event (11 January 1996). On the first two occasions, which represent periods of no freshwater influence, the vertical distribution of SSC was generally uniform, especially on spring tides. On the third occasion, runoff produced a slightly stratified water column but there was only a 20 mg l1 top–bottom difference in SSC. At this time, near-bed SSCs were always highest (generally the basal 0.5 m). In addition, turbidity profiles measured in three cross-channel sites at the mouth during the early flood of an intermediate tide (8 June 1994), showed a maximum cross-channel difference of only 10 mg l1. 3.2.4. Effect of waves on SSCs An analysis of wind data recorded at Townsville airport and SSCs at the lower creek site (for six turbidity datasets collected from July 1993 to February 1994; data not presented but available in Bryce, 2001) showed no correlation between winds and high SSCs. It is inferred here that an increase in local wave climate is a potential mechanism for re-suspending muddy sediments in the
Fig. 7. Current speed (measured at 1 m above the bed) at the lower creek site and near-bed SSCs (at 0.2 m above the bed) on the low-intertidal mudflat, lower creek and salt flat sites during a spring tide period where ebb tide speeds were fastest (10–16 December 1993). Numbers on current speed figure are the maximum high water elevations reached in m AHD. F, larger flood tide of the day; E, larger ebb tide of the day. Note the significantly smaller SSC scale used for the salt flat data in comparison to the other two sites.
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nearshore zone off Cocoa Creek (as found in other parts of Cleveland Bay—Larcombe et al., 1995). During the periods covered by the wind data, wind direction ranged from NE to SE and speeds rarely rose above 10 kn, thus representing typical background conditions during the wet and dry season, rather than conditions related to meteorological storm events. 3.3. Suspended sediment concentrations on the low-intertidal mudflat and salt flat On the low-intertidal mudflat, peak SSCs during within-channel tides rarely exceeded 150 mg l1. During some of the larger overbank tides, there was little or no turbidity registered on the overbank flood tide whereas SSCs of up to 400 mg l1 were observed on the subsequent ebb tide (Fig. 7). SSCs on the salt flat were low (Fig. 7), with a maximum SSC on the flood tide of up to 100 mg l1 on the largest overbank tides but generally 40–60 mg l1 (from observation of five spring tide periods). SSCs recorded on the salt flat during the ebb tide were generally <50 mg l1. 3.3.1. Suspended sediment fluxes in the channel Suspended sediment fluxes have been calculated at the lower creek site and are the product of the mean cross-sectional current and SSC. The calculations assume that suspended sediments are homogenous across the creek and throughout the water column, assumptions which are consistent with the SSC data described above and similar to the findings of Larcombe and Ridd
(1996) in nearby Gordon Creek (locality Fig. 1). At the lower creek site, the vertical velocity profile was derived using the Karman–Prandtl equation, which assumes a logarithmic vertical profile with the depth-averaged velocity occurring at a height of 0.38 of the flow depth above the bed. A roughness length of 1.0 cm was used, after applying the equation of Lettau (1969) to echosounder records of bedforms on the channel bed. Suspended sediment fluxes were calculated over: (a) a spring tide period in July 1993 (for 1-week period, faster ebb tide); (b) a spring tide period in September 1993 (for 1-week period, faster flood tide; and (c) a 3-week period in February 1994, (Fig. 8; faster flood tide see Fig. 3a). The average calculated error on the suspended sediment flux per tide is 21%, comprising 9% for the natural (random) variation in water volume at the mid-estuary site, and 12% for the cross-calibration of the nephelometers and conversion of turbidity data to SSCs. The average systematic error in calculating water volumes at the lower creek per tidal cycle (i.e. an average export of 30% of the total flood and ebb volume, as previously mentioned) was adjusted for in the calculation of sediment fluxes for each flood tide. For within-channel tides, net suspended sediment fluxes per tidal cycle are mostly landwards (Fig. 8), but on overbank tides, net fluxes are strongly seawards. For a complete spring–neap period, calculated net fluxes may be landwards or seawards—for the three datasets
Fig. 8. Measured net suspended sediment flux per tidal cycle at the lower creek locality (February 1994).
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outlined above, results are approximately 61 t seawards (for July) and approximately 465 and 91 t landwards (September and February, respectively). The estimated gross flux of sediment moved for these same datasets were 1936, 1282 and 3368 t for July, September and February, respectively. Taking into consideration the errors involved in calculating the net fluxes per tidal cycle (error bars shown in Fig. 8), the net movement of suspended sediment over a spring–neap period could be very small in either direction.
4. Discussion 4.1. Factors driving tidal asymmetry Tidal asymmetry in shallow estuaries takes the form of unequal durations and/or unequal magnitudes in the flood and ebb tide. It mostly results from the frictional interaction of the tide with the channel and intertidal areas (Friedrichs & Aubrey, 1988; Speer & Aubrey, 1985). In the literature, the type of tidal asymmetry present in a channel has been expressed using the following parameters: (1) the relative tidal amplitude (ratio of offshore M2 tidal amplitude (a) to channel depth at MSL (h) = a/h) and (2) the relative estuarine intertidal storage (ratio of volume of intertidal storage (Vs) to the channel volume at MSL (Vc) ¼ Vs/Vc). In estuaries where the fastest speeds are during the flood tide, tidal asymmetry is thought to be controlled by the a/h ratio, whereas flow in those estuaries with fastest speeds on the ebb are thought to be controlled by a large relative estuarine intertidal storage. For a/h < 0.2, an estuary would tend to have a fast and short ebb tide, and for a/h > 0.3, it would tend to have a fast and short flood tide (Friedrichs & Aubrey, 1988; Table 3). Ratios of the total tidal amplitude to channel depth at MSL were calculated for the lower creek site at Cocoa Creek over four spring tide periods (Table 3). The total tidal amplitude near the creek mouth was used instead of the M2 amplitude because the tidal wave may have already undergone some distortion on passing across Cleveland Bay (i.e. before it reaches Cocoa Creek). Most values of a/h at lower Cocoa Creek fall into the transitional to flood range of Friedrichs and Aubrey (1988) rather than consistently reflecting the measured tidal asymmetry, indicating that the relative tidal
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amplitude does not alone determine the tidal asymmetry in Cocoa Creek. To evaluate more fully the effect of external tidal forcing upon the creek (i.e. the nature of any offshore tidal asymmetry that may be imposed on the system), an analysis of predicted and measured tidal heights (for Townsville port for the period July 1993 to June 1994) was undertaken, and the ratio of the rate of flood rise to ebb fall for overbank tides calculated. Results fall mostly between 1.0 and 1.4 (Fig. 9), indicating that the regional tidal forcing is dominated by a tide with a relatively fast flood tide. The nature of this tidal forcing might partially explain the apparent offset between the calculated a/h results (mostly transitional to faster flood tide—see Table 3) and the measured asymmetry (i.e. Cocoa Creek demonstrates both faster flood and ebb tides at different times of the year), when compared to a/h results found elsewhere (Friedrichs & Aubrey, 1988). Wolanski, Mazda, and Ridd (1992b) associated faster ebb tidal speeds in mangrove creek systems to a high ratio of Vs/Vc, and for seven mangrove creek systems they noted that ratios of Vs : Vc ranged from 2 to 7 (with one exception of 44). This range is comparable to Cocoa Creek (Table 4). Wolanski et al. (1992b) reported the slopes of the swamp substrate were 1–4 103 (mostly 2–3 103), generally steeper than for Cocoa Creek (Table 4). These data indicate that the intertidal storage capacity of overbank tides at Cocoa Creek should be sufficient to repeatedly induce a relatively fast and brief ebb tide, but this is not always the case (e.g. Fig. 3). A potential explanation for this discrepancy is the regional tidal forcing (see above), but there may also be an additional mechanism. The slope of the ebb tide over the salt flats are often steeper than the salt flat substrate itself (1–2 103) so that areas closest to the creek drain
Table 3 Comparison of the Friedrichs and Aubrey (1988) ratio of relative tidal amplitude (a/h) findings (see text) to calculated a/h values for overbank tides at Cocoa Creek
Friedrichs and Aubrey (1988) Lower Cocoa Creek on overbank tidal (this paper)
Faster ebb tide
Faster flood tide
<0.2 0.21–0.23
>0.3 0.18–0.47
Fig. 9. Calculated ratios of rising rate of flood tide to falling rate of ebb tide for overbank tides at Cocoa Creek over a 12 month period. Data are from measured and predicted tidal heights at Townsville Port where the elevation of high water exceeds þ1.5 m AHD.
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Table 4 Measured substrate slopes and calculated estuarine intertidal storage ratios (Vs : Vc) at significant tidal elevations, Cocoa Creek Tidal elevation (m AHD)
Substrate slope
Intertidal storage volume : channel storage volume (Vs : Vc)
+1.85 (upper limit of salt flat) +1.60 (salt flat) +1.50 (overbank) +1.30 (mangrove fringe)
4.3 104
8.6
1–2 103 3.8 2 104–1 103 2.6 1101 1.7
first (Aucan, 1998). Later in the ebb, the upper salt flat areas slowly drain through small tributaries, resulting in a prolonged ebb flow. The tidal asymmetry of Cocoa Creek is thus dependent on the interaction between at least two controls: the offshore forcing of a shorter and faster flood tide, and the internal forcing of a faster ebb tide, imparted by the intertidal storage effect. This balance is significant because of its consequences for sediment transport. 4.2. Suspended sediment transport 4.2.1. Controls on SSC and suspended sediment fluxes The primary control upon SSCs and sediment transport in Cocoa Creek are the tidal currents. On timescales of hours, advection is a significant factor in the SSC variations, especially during overbank tides (Fig. 10). At most sites, on the flood tide, SSCs rise as mud is resuspended off the bed, but the exception is in mid-creek, where the bed is sandy (Bryce, 2001) and SSCs tend to peak only upon the arrival of turbid water from the lower creek. Later in the flood tide, with deepening water and slowing tidal currents, SSCs in the nearshore zone decrease, so that less turbid water is advected into the creek system. Over timescales of weeks, for example between two spring tide periods, the relative magnitude of SSCs on overbank tides appears to be, in part, controlled by the local availability of re-suspendable sediment (e.g. Fig. 6). Differences in SSC magnitudes do not appear to be due to the effects of wind under background conditions (i.e. through re-suspension of muddy nearshore sediment which is then available for import by tidal currents), although this may become more important during storm events. During all neap and intermediate tides (i.e. withinchannel tides), suspended sediment transport in the channel is controlled by the shorter and faster flood tide, in which suspended sediment fluxes are moved landwards. Greater sediment transport occurs during spring tide periods, where the direction and magnitude of net suspended sediment flux is influenced by: (a) Tidal asymmetry. For spring tide periods with faster flood overbank tides, the net landward movement of
suspended sediment is dependent on the total amount moved during within-channel tides. This is because the maximum current speeds of the overbank flood and ebb tides are of similar magnitude (Fig. 3a) and result in little net flux. In contrast, for spring tide periods with a faster overbank ebb tide, the export of suspended sediment is driven by an ebb tide which is far faster than the preceding flood flow (Fig. 3b). (b) Sediment availability. During neap tides net suspended sediment fluxes per tidal cycle tend to be below 20 t and the remaining within-channel tides rarely exceed 80 t (Fig. 11). Approaching the largest overbank tides (i.e. HW approximately þ1.85 m AHD), large amounts of suspended sediment are exported on the first few overbank tides (immediately prior to the largest tide itself), limiting the amount of suspended sediment available for export on subsequent large tides. This produces a wide variation in net suspended sediment fluxes at equivalent high tidal ranges (Fig. 11) and can be considered as a hysteresis effect in sediment concentration. 4.2.2. Implications for suspended sediment transport on annual and longer timescales During all neap and intermediate tides, suspended sediments are pumped towards the head of the creek, thereby providing a temporary local supply of sediment for potential transport on the first few overbank tides, some of which may be removed by overbank tides. SSC data from the salt flat (Fig. 7) suggests that, during overbank flood tides, some finegrained sediment is transported up onto the salt flat, while some may also be transported seawards on the subsequent ebb. In the latter part of the year (late September to early December) the relative absence of overbank tides indicates a sustained period of sediment accumulation in the head of the creek. Later (December–February), when the wet season coincides with spring tide periods of mostly faster ebb currents and seaward-directed net suspended sediment fluxes, this store of sediment might be exported seawards out of the creek. SSC data from the low-intertidal mudflat at the creek mouth suggests that on the ebb tide, suspended sediment is exported from the system (Fig. 7). We do not have reliable sediment transport data for the months of March–June, but we infer from the tidal characteristics that there will be periods of flood-directed and ebb-directed net suspended sediment flux. It is thus clear that the direction of net suspended sediment fluxes in Cocoa Creek is highly variable on timescales of months. On the timescale of years to decades, despite the large nature of our datasets it is not obvious as to whether Cocoa Creek is either a net importer or exporter of muddy sediment. SSCs from the salt flat suggest that
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Fig. 10. Plan view of the turbidity distribution (expressed as SSCs) in Cocoa Creek at various stages of an overbank tide on 18 January 1996, together with the main controlling factors. The tidal elevations expressed in AHD at the mouth and upper creek sites are taken from real depth values. The distribution pattern of SSCs is also based on turbidity data measured during a deployment of four nephelometers along the main channel (locations shown in Fig. 2a), each placed at approximately 0.2 m above the bed.
sediment is not being deposited on the salt flat, and therefore over geological time, the area in which marine water is held up at high tide, is not being reduced. This implies that large overbank ebb tides will still continue to scour the main channel and remove fine sediment that has been pasted on the walls of the channel during neap and intermediate tides, and including spring tide periods with faster flood tides. In addition, while land-based sources of muddy sediment to the Cocoa Creek tidal system are negligible, the shallow inner shelf of adjacent Cleveland Bay contains a virtually unlimited supply of mobile muddy sediments available to be transported into the small Cocoa Creek system (Carter, Johnson, &
Hooper, 1993). Geologically, the creek system is therefore not limited by sediment supply, and we conclude tentatively that the geomorphology of Cocoa Creek may be near a long-term equilibrium.
5. Conclusions The tidal asymmetry of Cocoa Creek is controlled by the interaction between offshore tidal forcing and the intertidal storage effect of the mangrove swamps and salt flats. The result is that there tends to be a predominance of either faster flood or ebb current speeds
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Fig. 11. Measured net suspended sediment flux per tidal cycle at the lower creek site of Cocoa Creek versus measured high-water elevation at Townsville Port (the closest port datum). Note that most seaward-directed suspended sediment fluxes occur on overbank tides. We have illustrated fluxes calculated for successive overbank tides of late February 1994 (spring period commencing in Fig. 3a). The data describe an open loop whereby tides of equivalent elevation (most notably where HW > þ1.85 m AHD) produce high net seaward suspended sediment fluxes early in the spring tide period, but up to 100 t less only a few tides later. m July 1993; } September 1993; d February 1994.
during certain periods of the year and that the nature of tidal asymmetry is therefore not the same for all overbank tides. For within-channel tides, a small amount of suspended sediment is transported mostly landwards, controlled by tidal asymmetry. Significant tidal suspended sediment transport in the channel is only initiated at overbank height (approximately þ1.5 m AHD) when resulting net fluxes are mostly seawards. On the larger overbank tides (where the maximum tidal height > þ1.85 m AHD) net sediment fluxes are reduced because of a limited supply of available material. The complex but strong relationships between hydrodynamics, sediment transport and geomorphology demonstrated in this and some other tropical tidedominated systems (e.g. Bryce et al., 1998; Larcombe & Ridd, 1995, 1996) indicates that complex sediment transport regimes are likely to be the rule rather than the exception. Given this complexity, and that the availability of sediment within these systems is limited at times, sediment fluxes should not be estimated for such systems using only sediment transport equations. This study also highlights the importance and necessity of using long time-series datasets to unravel linkages between sedimentary processes and the long-term sedimentary status of tidal mangrove creek systems.
Acknowledgements Many thanks go to Ross Hyne for assistance with fieldwork, and loan of a boat, vehicles and other field
equipment. Thanks are also due to Mick Fitzpatrick from the Townsville Port Authority for access to tidal data, Russell Jaycock from the Bureau of Meteorology for the wind data and Gregg Brunskill of the Australian Institute of Marine Science for access to AUSLIG topographic maps. This work was funded by an ARC Large Grant to P.L. and P.V.R., and by James Cook University. The two anonymous journal reviewers are thanked for their helpful comments in improving the manuscript.
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